Henry Sweet (1877) A Handbook of Phonetics, p 174–175: "Even letters with accents and diacritics [...] being only cast for a few founts, act practically as new letters. [...] We may consider the h in sh and th simply as a diacritic written for convenience on a line with the letter it modifies."
Nestle, Eberhard (1888). Syrische Grammatik mit Litteratur, Chrestomathie und Glossar. Berlin: H. Reuther's Verlagsbuchhandlung. [translated to English as Syriac grammar with bibliography, chrestomathy and glossary, by R. S. Kennedy. London: Williams & Norgate 1889].
Coakley, J. F. (2002). Robinson's Paradigms and Exercises in Syriac Grammar (5th ed.). Oxford University Press. ISBN 978-0-19-926129-1.
Michaelis, Ioannis Davidis (1784). Grammatica Syriaca.
S.P. Brock, "An Introduction to Syriac Studies", in J.H. Eaton (Ed.,), Horizons in Semitic Studies (1980)